I've been cogitating on this and would love to hear from someone more immersed in Japanese culture than myself.

The question is, how would a senior Japanese teacher view this situation? I could believe that the correct, accepted behavior for a Japanese student in this situation would be to go train with whomever he wanted and not tell the teacher about it. The reason being that the teacher has made his position known; for the student to argue with the teacher about it would suggest that the student knows better or that the teacher was wrong and should make an exception; by not telling the teacher, the student acknowledges the teacher's right to make such restrictions; and that as long as no one tells anybody anything about it, everybody can go on pretending it's not happening and everyone's honor is satisfied.